1. Field of the Invention
This invention relates generally to laser Doppler velocimeters of the type used to measure a localized fluid velocity, and more particularly, to methods and apparatus to calibrate laser Doppler velocimeters.
2. Description of the Prior Art
Laser Doppler velocimeters are used to optically measure the velocity of a fluid without disturbing the flowfield. The basic principle of the operation is the laser light scattered from entrained particles in a moving fluid will be Doppler shifted by an amount determined from the laser wavelength, the transmission and collection geometry, and the velocity of the moving fluid. Since the frequency of this scattered light is directly proportional to the velocity of particles at the scattering region, unknown velocities can readily be obtained once a calibration factor, or factors for a multidimensional velocimeter, for the velocimeter has been derived.
One conventional method of determining calibration constants for laser Doppler velocimeters is by direct calculation given the geometric relationships of the system. Such calculations can be accurate for extremely simple systems but become increasingly more complicated and less reliable as the complexity of the system increases. In complex laser Doppler velocimeter systems, for example, where the velocity is measured in three dimensions, or where zoom optics are employed, calculations from geometric considerations alone are impractical and give poor results.
Another calibration method involves generating a reference flowfield having a precisely predetermined velocity and directing it through the measuring volume of a laser Doppler velocimeter. While conceptually this method could result in extremely accurate calibrations, in practice the flow must be controlled by sensitive pressure regulators and transducers which may themselves have uncertain calibrations.
Another approach to calibrating laser Doppler velocimeters is to utilize a simulated flow. A spinning disk with a rough surface has been employed for such a simulation purpose. The rotating rough surface simulates the passage of small particles to the impinging velocimeter beam. A spinning disk which uses its moving surface to generate a velocity signal has two significant disadvantages. First, the signal is often distorted resulting in ambiguous zero crossings which are difficult to process. Second, the disk does not work well when impinging velocimeter beams are angles other than normal. This makes the calibration of a three-dimensional system difficult because it is necessary to obtain multiple velocity components.
The following references, all U.S. patents, describe various rotating objects used for scientific applications, only one of which applies to the calibration of a laser Doppler velocimeter. The calibrator described in that particular reference does not make any use of a spinning disk.
U.S. Pat. No. 4,148,585, issued to Bargeron et al. teaches the use of a spinning disk defraction grating in a laser Doppler velocimeter device to produce several beams of different frequency.
U.S. Pat. No. 4,154,669, issued to Goetz teaches the use of a rotating defraction grating to modulate the laser Doppler source for production of an automatic recording of mobility in an electrophoresis apparatus.
U.S. Pat No. 4,176,950, issued to Franke discloses the calibration of a laser Doppler velocimeter by the use of a rotating transparent block that refracts the light as it rotates.
U.S. Pat. No. 4,190,367, issued to Hard af Segerstad et al. teaches an apparatus for measuring the surface roughness of an object by reflecting a laser beam from its surface through a rotating defraction grating into a photodetector.
U.S. Pat. No. 4,227,807, issued to Pond et al. discloses an interferometer that uses a spinning reticle to produce a plurality of defraction beams.
U.S. Pat. No. 4,311,383, issued to Ohtsubo discloses a method of determining velocity by relying on the "speckle patterns" created by a laser reflecting from a spinning disk with surface roughness.